CVE-2026-18738 in Shlink
Summary
by MITRE • 08/04/2026
Shlink versions 5.0.0 through 5.1.5 contain a CSV formula injection vulnerability that allows unauthenticated remote attackers to plant spreadsheet formulas into exported visit data by supplying malicious values in User-Agent, Referer, or request path headers beginning with formula-triggering characters such as =, +, -, or @. Attackers can craft a single unauthenticated request against any short URL to embed DDE or WEBSERVICE formula payloads into CSV cells, which are then executed on an administrator's client machine when the exported CSV file is opened in a spreadsheet application that evaluates formulas.
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Analysis
by VulDB Data Team • 08/04/2026
This vulnerability exists within the Shlink URL shortening platform versions 5.0.0 through 5.1.5 where the application fails to properly sanitize user input during CSV export operations. The flaw stems from inadequate validation of headers including User-Agent, Referer, and request path parameters that are incorporated into exported visit data. When these headers contain formula-triggering characters such as equals signs, plus signs, minus signs, or at symbols at the beginning of their values, they are directly embedded into CSV cells without proper escaping or sanitization. This represents a classic csv injection vulnerability that aligns with CWE-1236, which specifically addresses improper neutralization of special elements used in CSV files. The vulnerability operates through a server-side data processing flaw where the application treats user-supplied header values as trusted input for export operations rather than validating and sanitizing them against formula execution patterns.
The operational impact of this vulnerability is significant as it enables unauthenticated remote code execution through social engineering tactics. Attackers can craft malicious requests targeting any short URL within the system, embedding dangerous spreadsheet formulas such as DDE (Dynamic Data Exchange) or WEBSERVICE payloads that automatically execute when the exported CSV file is opened in Microsoft Excel, Google Sheets, or other spreadsheet applications that evaluate formulas by default. The attack requires no authentication and can be executed against any accessible short URL, making it particularly dangerous for administrators who regularly review visit data exports. When victims open the malicious CSV files in their spreadsheet applications, these formulas execute automatically, potentially downloading malware, executing commands, or exfiltrating data from the victim's system. This vulnerability directly maps to ATT&CK technique T1059.006 for execution through Microsoft Excel and T1566 for social engineering via malicious documents.
Mitigation strategies should focus on implementing proper input sanitization and output escaping mechanisms within the CSV export functionality. The application must validate all header values before incorporating them into exported data, specifically checking for formula-triggering characters at the beginning of strings and either escaping them with a single quote prefix or rejecting such inputs entirely. Organizations should also implement network-level protections such as disabling automatic formula evaluation in spreadsheet applications, configuring security policies to prevent execution of external commands from spreadsheets, and restricting access to sensitive CSV export functionality through authentication mechanisms. Additionally, regular input validation should be implemented across all user-supplied headers that may influence exported data formats, ensuring that any potentially dangerous characters are properly neutralized before data is written to output files. The fix should align with secure coding practices outlined in OWASP Top 10 and the CWE top 25 most dangerous software weaknesses to prevent similar vulnerabilities from occurring in future development cycles.